Be more lenient about non-existent pluginDirs.
Be more lenient about non-existent pluginDirs.

ProgVal/Limnoria-test,kblin/supybot-gsoc,Ban3/Limnoria,prashantpawar/supybot-rothbot,buildbot/supybot,ProgVal/Limnoria-test,frumiousbandersnatch/supybot-code,mazaclub/mazabot-core,raboof/supybot,haxwithaxe/supybot,mazaclub/mazabot-core,jeffmahoney/supybot,Ban3/Limnoria